1. Field of the Invention
The present invention relates to a toner for use in copiers, facsimiles and printers and the like using electrophotographic image forming methods; a two-component developer using the toner; and an image forming apparatus using the two-component developer.
2. Discussion of the Background
The electrophotographic image forming method includes a charging process charging a surface of a photoreceptor which is an image bearer with an electric discharge, an irradiating process irradiating the charged surface of the photoreceptor to form an electrostatic latent image, a developing process developing the electrostatic latent image formed on the surface of the photoreceptor with a toner to form a toner image, a transfer process transferring the toner image on the surface of the photoreceptor onto a surface of a transfer body, a fixing process fixing the toner image on the surface of the transfer body and a cleaning process removing the toner remaining on the surface of the image bearer after the transfer process.
Recently, color image forming apparatuses using the electrophotographic image forming method are widely used, and digitalized images are available with ease and printed images are required to have higher image definitions. While higher image resolution and gradient are studied, the toner visualizing the latent image is studied to have further sphericity and smaller particle diameter to form a high definition images. As the toner prepared by pulverizing methods has a limit of these properties, polymerized toners prepared by suspension polymerizing methods, emulsification polymerizing methods and dispersion polymerizing methods capable of conglobating the toner and making the toner have a small particle diameter are being used.
The toner having a shape close to a true sphere is easily affected by a line of electric force in an electrostatic developing method and is faithfully developed along the line of electric force of an electrostatic latent image on a photoreceptor. When a minute latent image dot is reproduced, the toner are precisely and uniformly located to have a high thin line reproducibility. In an electrostatic transfer method, as the toner has a smooth surface and a good powder fluidity, the toner particles less adhere each other and to the photoreceptor, and therefore the toner is easily affected by a line of electric force and is faithfully transferred along the line of electric force, i.e., the toner has a high transferability.
However, the toner having a shape close to a true sphere has a smaller surface area than an amorphous toner, i.e., has less surface area which can effectively used for frictional charge by a magnetic carrier and friction charging members such as developer regulating members. The spheric toner easily slip on a surface of the friction charging member and charged speed and level thereof decrease, and therefore a specific amount or more of a charge controlling agent is needed therefor.
In addition, as the toner having a smaller particle diameter to improve minute dot reproducibility has a larger superficial area, and an external additive is used in a large amount. Since the external additive largely changes frictional chargeability of the toner, it is essential for the toner to have chargeability, developability and transferability.
Japanese Laid-Open Patent Publication No. 11-184145 discloses a developer comprising a toner comprising a binder resin and a colorant, a particulate silica and a particulate resin, wherein the particulate silica is a mixture of a first particulate silica and a second particulate silica having a different number-average particle diameter each other and present in an amount of 0.1 to 3.0% by weight per 100% by weight of the toner, the particulate resin is present in an amount of 0.01 to 0.1% by weight per 100% by weight of the toner, the first particulate silica having a smaller particle diameter relative to the second particulate silica has a number-average particle diameter less than 15 nm, the second particulate silica having a larger particle diameter relative to the first particulate silica has a number-average particle diameter of from 15 nm to 150 nm, and a ratio of the number-average particle diameter of second particulate silica having a larger particle diameter relative to the first particulate silica to that of the particulate resin is from 0.05 to 20. However, this method simply adds a mixture of the silica and particulate resin to an external of the toner, and which will not have stable chargeability for long periods.
Japanese Laid-Open Patent Publication No. 2000-292978 discloses a toner comprising a low-molecular-weight resin, a polymer resin and a colorant, wherein the polymer resin is eccentrically-located adjacent to a surface of the toner, and preferably a particulate release agent is also eccentrically-located adjacent thereto. This provides a polymerized toner having hot offset resistance and good chargeability, and preventing a transfer sheet from being entwined around a fixer fixing a toner image upon application of heat, and a method of preparing the toner. However, the toner will not have stable chargeability for long periods.
Because of these reasons, a need exists for a toner having stable chargeability and fluidity even after used for long periods in an image developer.